The present invention relates to a method for producing superconductive wires of multifilaments which are encased in copper or a copper alloy and contain niobium and aluminum.
The basic structure of superconductive, copper stabilized Nb.sub.3 Al wires utilized in the past has been a pure copper matrix in which a plurality of Nb.sub.3 Al filaments (7 or 19) was embedded (See Ceresara et al, IEEE Transactions of Magnetics, Volume. MAG-15, No. 1, January, 1979, pages 639 to 641). In this structure, each filament was produced according to the so-called "Swiss roll" or "French roll" in which thin foils of niobium and aluminum are alternatingly tightly wound around a copper cylinder and the total structure is drawn in the direction of the roll axis into thin wires having a diameter of about 0.2 mm. Seven or nineteen of such wires are inserted into corresponding bores of a larger copper cylinder and the entire unit is drawn again until a final outer diameter of about 0.45 mm is obtained.
The final heat treatment to form the Al5 compound Nb.sub.3 Al is effected at a temperature of about 1173.degree. K. By producing wires in this manner, it is possible to achieve a critical current density on the order of magnitude of 2.times.10.sup.5 A/cm.sup.2 at 64 kG and a transition temperature near 16.degree. K.
However, this process has serious disadvantages in that it involves a very complicated technique, and requires very thin aluminum and niobium foils whose dimensions are limited by the manufacturing process itself. Moreover, due to the helical geometry of the bands which are processed into wire, the coils constructed for superconductive magnets will most likely require long excitation periods.
It has also been proposed to bring niobium and aluminum powder into very intimate, mechanical contact in a high energy ball mill by using heavy balls, thus causing, in the course of the process, the powder particles to be quasi-welded together and, further causing the existing or resulting particles to be broken up or to be comminuted. See Larson et al., Manufacture of Superconducting Materials, Proc. of an International Conf., Nov. 8-10, 1976, Port Chester, N.Y. (1977) pp. 155-163. It has been found that the effective thickness of these treated powders is reduced and the process continued until the particle size and/or thickness fall substantially below 1 micron. Thereafter, the alloy powder is packed into a tantalum tube for the production of wire, the tantalum tube is inserted into a copper tube and its entire unit is drawn into a wire. The Al5 phase is realized by heating the alloy powder to 1123.degree. K.
With this process, called "mechanical alloying," the niobium particles are deformed to a great and uncontrollable (localized) extent. Thus, these particles are damaged and become very hard, and therefor, no wire of kilometer length can be produced from this material without the occurrence of breaks.
In the past, only Nb.sub.3 Sn wires could be used for the production of superconductive magnets up to 15 tesla while V.sub.3 Ga, which can be used for higher telsa values, is uneconomical for the production of large superconductive magnets due to its gallium component and the high manufacturing costs connected therewith.